Comparison of the tamoxifen regulated chimeric Cre recombinases MerCreMer and CreMer

The site-directed recombinase Ore can be employed to delete or assemble gen es in the genome of living cells. We have constructed expression vectors fo r chimeric Cre recombinases carrying a mutated hormone binding domain eithe r at the C-terminus only (CreMer) or at both the N and C-termini (MerCreMer ). The chimeric Ore proteins can be activated by culturing transfected cell s with 4-hydroxytamoxifen. In transfected embryonic stem (ES) cells, we com pared the extent of recombination of a flexed gene with the expression leve ls of the chimeric Ore proteins. Our data demonstrate that the bulky MerCre Mer protein is not less active than the CreMer protein. Thus, the tighter c ontrol and low background activity of the MerCreMer enzyme is not due to a generally low recombinase efficiency.